Mo‐O‐C Between MoS2 and Graphene Toward Accelerated Polysulfide Catalytic Conversion for Advanced Lithium‐Sulfur Batteries

Abstract MoS2/C composites constructed with van der Waals forces have been extensively applied in lithium–sulfur (Li–S) batteries. However, the catalytic conversion effect on polysulfides is limited because the weak electronic interactions between the composite interfaces cannot fundamentally improve the intrinsic electronic conductivity of MoS2. Herein, density functional theory calculations reveal that the MoS2 and nitrogen‐doped carbon composite with an Mo–O–C bond can promote the catalytic conversion of polysulfides with a Gibbs free energy of only 0.19 eV and a low dissociation energy barrier of 0.48 eV, owing to the strong covalent coupling effect on the heterogeneous interface. Guided by theoretical calculations, a robust MoS2 strongly coupled with a 3D carbon matrix composed of nitrogen‐doped reduced graphene oxide and carbonized melamine foam is designed and constructed as a multifunctional coating layer for lithium–sulfur batteries. As a result, excellent electrochemical performance is achieved for Li–S batteries, with a capacity of 615 mAh g–1 at 5 C, an areal capacity of 6.11 mAh cm–2, and a low self‐discharge of only 8.6% by resting for five days at 0.5 C. This study opens a new avenue for designing 2D material composites toward promoted catalytic conversion of polysulfides.


Preparation of CF-rGO precursor
4 g ammonium bicarbonate was dissolved in 75 mL deionized water, and then 300 mg graphene oxide was added and dispersed by sonication. The melamine foam was rinsed with alcohol to remove some contaminants and dried at 60 ℃ for 2 h. The dried melamine foam put into the above graphene oxide aqueous solution to soak and then dried through freeze-dry.
Subsequently, the dried foam was calcined at 600 ℃ for 2 h with a heating rate of 5 ℃ min -1 under pure Ar flow. After that, the 3D porous nitrogen doped reduction of graphene oxide supported on carbon foam (CF-rGO) precursor was obtained.

Preparation of MoS 2 @CF-rGO
To fabricate the MoS 2 @CF-rGO, a piece of as-prepared CF-rGO precursor was first immersed in a teflon-lined stainless-steel autoclave containing a mixed solution with 60 mg thiourea and 30 mg sodium molybdate, which kept at 200 ℃ for 24 h. After the hydrothermal treatment, the sample was soaked in deionized water to remove the residual impurities and dried through freeze-dry to obtain the MoS 2 @CF-rGO precursor. Finally, the precursor was annealed at 600 ℃ for 2 h under Ar gas to get MoS 2 @CF-rGO. Meanwhile, for comparison, the NRGO supported on graphitic carbon foam (CF-rGO) was obtained by annealing CF-rGO precursor at 600 ℃ for 2 h with a heating rate of 5 ℃ min -1 under pure Ar flow. In additional, in order to prove that nanoflower-like MoS 2 has abundant edge site, the 60 mg thiourea and 30 mg sodium molybdate was dissolved in 30 mL deionized water, and then put into a teflon-lined stainless-steel autoclave and keep at 200 ℃ for 24 h. After MoS 2 microsphere was obtained by further annealed at 600 ℃ for 2 h under Ar gas, as shown in Fig S10, mark as pure MoS 2 . Meanwhile, the pure MoS 2 and CF were mixed in deionized water and dried to obtain a physically mixed compounds (marked as MoS 2 -CF-NRGO), thus to demonstrate the enhancement of Mo-O-C band on the catalytic activity of the material.

Fabrication of function separator
The obtained MoS 2 @CF-rGO or CF-rGO and PVDF (9:1 by mass) were uniformly dispersed in N-methylpyrrolidone (NMP) by sonication for 30 min. The mixed solution was further coated on a polypropylene (PP) separator (Celgard 2325) by using a vacuum filtration method, and then vacuum dried at 60 ℃ for 12 h. Finally, the obtained modified separator was punched into disks with a diameter of 19 mm for the separator of lithium-sulfur batteries.

Adsorption tests of polysulfides
The Li 2 S 6 with a concentration of 3 mmol L -1 were prepared by mixing S powder and Li 2 S at molar ratio of 5:1 in a mixed solvent 1,2-dimethoxyethane (DME) and 1,3-dioxolane (DOL) (v/v=1:1) under magnetic stirring for 48 h. 10 mg of MoS 2 @CF-rGO and CF-rGO was introduced into the Li 2 S 6 solution (5 mL), respectively, followed by aging for 24 h.

Fabrication of Li 2 S 6 symmetric cells and measurements
Functional materials (MoS 2 @CF-rGO or CF-rGO) and PVDF binder with a weight ratio of 9:1 was uniformly mixed into the NMP and then coated onto the aluminum foil. After dried at 60 °C for 12 h and punched into disks with a diameter of 12.0 mm, the areal loading of the obtained electrode was about 1 mg cm −2 . Then, the identical working and counter electrodes of MoS 2 @CF-rGO, CF-rGO with a PP separator, and 40 μL electrolyte containing 0.5 M Li 2 S 6 in a mixed solvent DME and DOL (v/v=1:1) were fabricated into the symmetric cells.
CV tests were carried out with a CHI660E electrochemical working station. The CV curves were recorded at a scan rate of 10 mV s -1 with in the voltage range of -1.5 -1.5 V.

Nucleation and dissolution of Li 2 S test
A Li 2 S 8 solution (0.5 mol L -1 ) were prepared by mixing S powder and Li 2 S at molar ratio of 7:1 in tetraglyme solution under magnetic stirring for 48 h. Similarly, 12 mm diameter aluminum foil coated with functional materials (MoS 2 @CF-rGO or CF-rGO) and PVDF binder with a weight ratio of 9:1 was used as working electrode to assemble the cell and a lithium foil was used as the anode. During the cell assembly process, 10 μL

Li-Li symmetric cells
The electrodes were prepared by Li metal foils with diameter of 12 mm, the PP separator and MoS 2 @CF-rGO, CF-rGO modified separator were used as separators, and using 40 μL Li-S battery electrolyte, texted at 2 mA cm -2 .

Assembly of Lithium-sulfur batteries and electrochemical measurements
Sulfur cathode was prepared via mixing sublimed sulfur, acetylene black and PVDF binder with a weight ratio of 6:3:1 was mixed in NMP to form a homogeneous slurry. The slurry was coated onto aluminum foil, followed by drying at 60 °C for 12 h in a vacuum oven and punched into discs with a diameter of 12 mm, the sulfur loading is about 1-1.2 mg cm −2 .
Sulfur/carbon cathode was prepared via mixing sublimed sulfur and acetylene black with a weight ratio of 8:2. Subsequently, the uniformly mixed powder was heated at 155 °C for 12 h in Ar-filled autoclave. The obtained S/C composite mixing acetylene black and PVDF binder with a weight ratio of 8:1:1 was mixed in NMP to form a homogeneous slurry. The slurry was coated onto carbon cloth (WOS1009), followed by drying at 60 °C for 12 h in a vacuum oven and punched into discs with a diameter of 12 mm, the areal sulfur loading is about 2-10 mg cm -2 . After that, standard coin cell (CR2025) with S or S/C cathode, Li anode, and functional separator was assembled in Ar-filled glove box. The electrolyte was consisted of 1.0 mol L -1 of bis(triuoromethane) sulfonimide lithium (LiTFSI) and 0.2 mol L -1 lithium nitrate (LiNO 3 ) dissolved in DOL/DME (v/v =1/1). The electrolyte/sulfur ratios are around 30 uL mg -1 for the cathodes with 1-1.2 mg cm -2 sulfur loading, 10 uL mg -1 for the cathodes with 2.0-10 mg cm -2 sulfur loading. The galvanostatic charge/discharge performance tests and the rate capability at different C-rates were performed using a Neware battery test system. Cyclic voltammetry (CV) measurements and electrochemical impedance spectroscopy (EIS) were carried out using a CHI660E electrochemical working station. The EIS was measured in the frequency range of 0.01-10 5 Hz.

In situ Raman spectroscopy
SIBs/Li-S batteries with a quartz window, provided by Beijing Science Star Technology Co. Ltd, were used for in situ micro-Raman spectroscopy analysis. For SIBs, the electrode slurry was pasted onto nickel foam after stirring and tailored to a disk after being heated in a vacuum overnight. The conditions for assembling the battery are consistent with the button batteries. For Li-S batteries, the same method was used to prepare cathodes as button batteries.
A hole was created on lithium metal foil to allow the laser shed on the separator. The cells were run at a (dis)charging rate of 50 mA g -1 (SIBs) and 0.5 C (LSBs). Raman signals were recorded simultaneously by a 532 nm laser.

Calculation method
All the calculations were performed based on the density functional theory used the Perdew-Burke-Ernzerh exchange-correlational functional of generalized gradient approximation, [1] and the projector-augmented wave method, [2] implemented by Vienna Ab-initio Simulation Package (VASP). [3] The cutoff energy for the plane wave-basis expansion was set to 500 eV and the atomic relaxation was continued until both the force acting on atoms was smaller than 0.01 eV Å -1 and the energy was converged to 1×10 -5 eV at       In order to prove that hydrothermal treatment has no effect on